Process of developing photolithographic printing plates



Patented Oct. 26, 1954 T OFFICE PROCESS OF DEVELOPING PHOTOLITHO GRAPHICPRINTING PLATES Frederic E. Brinnick,

Westbrook, Maine, assignor to S. D. Warren Company,

Boston, Mass, a corporation of Massachusetts No Drawing. ApplicationApril 23, 1951, Serial No. 222,523

8 Claims.

In the practice of photolithography a suitable base such as a grainedmetal plate or a suitably prepared (e. g., mineral-coated) paper sheethaving a hydrophilic surface and bearing a watersoluble photosensitivecoating which becomes water-insoluble and more or less hydrophobic uponexposure to actinic radiation is imaged by exposure to actinic radiationand is then developed. The development serves to remove or wash oil theunexposed areas of the photosensitive, coating and to leave such areaswet with water and repellent to ink and to leave the exposed areas orimage repellent to water and wettable by ink. Exposed photosensitiveplates have been developed heretofore generally by one or the other ofthe following methods depending upon the character of the plate.

(a) The exposed plate may be first coated with developing ink and thenscrubbed with water to remove the unexposed areas of the photosensitivecoating together with the ink adhering thereto and to leave such areaswet with Water, the exposed areas with the adhering ink remainingunalfected by the scrubbing treatment. The sodeveloped plate may then beprinted by being contacted successively with a wetting roller, an inkingroller and the surface to-be printed. In the printing process the inkedimaged areas will continue to take ink from the inking roller and torepel water and the wetted unimaged areas will continue to take waterfrom the wetting roller and to repel ink. This is the customaryprocedure with plates having the conventional photosensitive layer ofalbumin and ammonium bichromate.

(b) The exposed plate is simply washed with water to remove theunexposed areas of the photosensitive coating and then put into theprinting machine where it is contacted successively with a wettingroller, an inking roller and the surface to be printed. In this processthe plate may be somewhat slow to take ink on the imaged areas so thatgood prints will not be produced immediately and possibly not untilafter the machine has run for a considerable length of time during whicha number of defective prints will have been made.

The albumin-dichromate plates are short lived and must be freshlyprepared within a few hours before they are used. Besides the developingprocess described is time-consuming.

Recently photosensitized planographic printing plates have appeared onthe market comprising a base of coated paper or other material andbearing on the surface a photosensitive layer composed of a highmolecular weight water-soluble diazo compound which will becomeinsoluble under the action of actinic radiation. An important advantagepossessed by such plates in comparison with conventional albumin plates.is that they have a comparatively long shelf-life, so that they can bepurchased in sensitized condition by the ultimate user and stored for aconsiderable time before actual use. The length of time plates of thistype can be stored without apparent deterioration may vary from a fewweeks to more than a year, depending upon the particular diazo compoundused as sensitizer.

In some cases, and especially in the case of plates capable of longstorage, it happens that the plate after exposure and washing with watermay be slow to take ink on the imaged areas.

It appears that when the exposed plate is developed by method (22) theexposed areas become to some extent wet with water and as a result repelink until the water gradually is replaced by ink.

The method commoly used for the development of the albumin-bichromateplates is not suitable for the development of the diazo plates for thefollowing reason. The albumindichromate coating is relatively thickamounting to one pound or more per 1,000 square feet of surface so thatthe developing ink applied to .the exposed plate does not penetratethrough the coating to the supporting base and is entirely removed fromthe unimaged areas by the scrubbing with water leaving the unimagedareas clean. In the diazo plates, on the other hand, the coating isrelatively thin amounting to only from one to several ounces per 1,000square feet. Consequently, if a conventional developing ink is appliedto the exposed plate it will penetrate through the coating to thesupporting base and will not be cleanly removed from the unexposed areasby scrubbing with water. As a result both the exposed and the unexposedareas will take ink and give defective prints.

The object of my invention therefore is to provide a method for thedevelopment of plates having diazo photosensitive coatings, which methodshall avoid the objectionable features of the prior development methodsreferred to above and will render the imaged areas of the plateimmediately wettable by ink while leaving the unimaged areas clean andreceptive to water and consequently repellent to ink.

As diazo compounds suitable for the new process there can be used diazocompounds of a Ar means an aromatic residue, and X an equivalent of ananion of an acid.

Under the term aroy I understand the acyl radicals of aromaticcarboxylic acids, e. g. benzoyl, and the expression equivalent of ananion of an acid is intended to designate radicals such as Cl, SO4/2,ZnCl4"/2, CeHsSOs, and the like.

The substituent Y and the diazo group N2 may preferably be in apara-position to each other. The aromatic residue Ar, the group R andthe NI-I-group may contain further substituents. In most cases it isadvantageous if the residues R. and Ar are substituted by alkoxy, aroxyor N- aryl-sulfamido groups or by one or more halogen atoms. When usinga diazo compound of the formula as above defined the substituents R andR1 can be connected with one another to form a cyclic compound, or R orR1 can be connected with Ar by a covalent linkage, but it is notdesirable to have sulfo groups as substituents in any of R, R1 and A1.

Diazo compounds which are obtained by treating the diazo compoundsmentioned above with carbonyl compounds such as formaldehyde may beused. Thus higher condensed diazo compounds are obtained which areespecially valuable for the new process.

The so-called diazo sulfonates which can be obtained by treating thediazo compounds or the aldehyde-diazo compound reaction products withalkali metal sulfites also may be used. Further diazo-amino-compoundswhich can be obtained by causing the diazo compounds in question toreact with guanidine, taurine or simi ar compounds may be used. Thelight sensitivity of these condensation products is usually weaker thanthat of the before-mentioned products and the shelflife of plates coatedtherewith is correspondingly longer.

Typical diazo compounds which when applied to paper and exposed toactinic radiation yield images which are benefited by treatmentaccording to the invention include the diazo compounds of 4 (N benzyl Nethyl) amino aniline, 4 (N 2,6 dichlorobenzyl) amino-aniline, 4-(N-cyclohexyl) -amino-aniline, 4-amino-2,5,4'- tribromo diphenylamine, 4amino2,4,6-trichlorodiph'enylamine, 4-amino 2 [N-(2,5-diethoxy-phenyl)sulfamido] diphenylamine, 4- amino-3,6-dimethoxy-diphenylamine 2carboxylic acid, 1-amino-2,5-di-n-propoxy-4-methyl diphenylsulfide, N(2,6-dichloro-benzyl) -3- amino-carbazol, 4 (N 2,3,4,6tetrachlorobenzyl) amino aniline, 4 (N 2,6 dichlorobenzyl, N-ethyl)-amino-2,5-diethoxy-aniline and 4-amino-2,5,4-triethoxy diphenyl-ether.

My invention is applicable to any diazo image which is not sufiicientlywater-repellent but is more particularly adapted for use with diazoimages derived from the sulfonates formed by reacting an alkali metalsulfite, e. g. sodium sulfite, with the reaction products of aldehydessuch as formaldehyde with diazo compounds of the type defined above,said sulfonates generally giving plates having a relatively longshelf-life but yielding images which tend to be deficient inwater-repellence.

In order to provide the paper base with a layer of the light sensitivediazo compounds there can be used solutions or suspensions of the diazocompounds which are applied to the paper surface by coating, spraying,or by means of appliancerollers or brushes or a pad. A surplus may beremoved by doctor knives, by blowing with air, by flowing or bycentrifuging. The coating solution should have a neutral or weaklyacidic reaction.

In practicing my present invention, I provide a suitable base, such as acoated paper base and preferably a water-impervious paper base, with athin photosensitive layer of diazo-compound of the type previouslymentioned; I expose the sensitized surface through a transparency to theaction of actinic radiation; I treat the exposed surface withnon-penetrating, hydrophobic, olephilic material and thereby render theexposed or imaged areas water-repellent; and I Wash the entire surfacewith aqueous liquid to clean the unexposed areas and provide a plateready for use as a printing plate on a planographic printing press orrotary offset duplicator.

The non-penetrating, hydrophobic material used to promotewater-repellency of the imaged areas may be (1) a finely divided powderor dust, such as finely powdered heavy metal soap or nonwater-absorbentdyestuff; or it may be (2) an oily material of at least mediumviscosity, such as oleic acid; or advantageously it may be a solution orsuspension of (l) in (2).

Typical metal soaps suitable for use according to the invention includezinc stearate, aluminum palmitate, calcium oleate, and the like. Typicalhydrophobic dyestuffs include Oil Red, Oil Orange, Hansa Yellow, and thelike. Watershedding powdered material of the type indicated may beapplied to the surface of an exposed plate by means of a soft brush or apledget of cotton. It is found that hydrophobic powder so appliedadheres firmly to the exposed or imaged areas of the plate. When theso-treated plate is subsequently washed or wiped with a sponge dampenedor Wet with water, the unexposed areas are easily wiped clean of boththe still soluble unexposed diazo-sulfonate sensitizer and any overlyingwater-repellent powder which may be adhering thereto. It will be notedthat during the cleaning or washing operation the image areas, to whichthe water-repellent powder particles adhere, are not Wet by the washwater because the water is repelled by the said water-repellent orwater-shedding particles. Likewise the image is not wet by subsequentlyapplied aqueous lithographic wet-out or dampening solution. Consequentlywhen the plate is put on an offset printing press the image is still dryand in condition to pick up ink upon first contact with the inkingrolls,thus avoiding loss of time hitherto required for ink to displace wateron untreated images which have adsorbed water or aqueous lithographicsolution.

Alternatively, instead of a water-repellent powder, there may be used anon-penetrating, water-repellent, oily liquid, such as oleic acid, anyof various vegetable oils having viscosities approximating that of oleicacid or liquid silicones having viscosities at least as high as that ofoleic acid. In fact, oxidized or polymerized turpentine, ifsufificiently viscous, can be used successfully. However, in the use ofthe latter substance care must be taken that the oxidation orpolymerization has proceeded far enough that the material will notpenetrate through the diazosulfonate coating to contaminate theunderlying base. To treat the plate with the hydrophobic liquid a swabwet with oleic acid or similar oily material is wiped over the exposedsurface of the plate, whereby the imaged areas are wet by the said oilymaterial and consequently are rendered non-water-wettable. When thesurface is subsequently wiped with a sponge wet with water theunexposed, still soluble sensitizer is removed together with anyoverlying film of oily material whereas the insoluble, imaged areasremain with a film of the water-repellent, oily material there on.Thereafter it is found that the oil-treated image immediately picks upink from an inkingroll passed thereover, so that prints can be madetherefrom without delay. In place of the oleic acid mentioned there maybe substituted with very similar results olive oil, cotton-seed oil,rapeseed oil, peanut oil, or the like.

It is frequently found advantageous to dissolve an oil-soluble dyestuifin the oleic acid or other oil used according to the invention. Theresulting solution makes the image on the plate more readily Visiblebefore it is inked, and probably likewise makes it slightly morewater-repellent. Such dyestufis are usually only sparingly soluble inoleic acid or vegetable oils, and may in general be added up to thelimit of their solubility. Alternatively or in conjunction, smallquantities of water-insoluble metallic soap may be dissolved in thehydrophobic oily material, thereby increasing the viscosity anddecreasing the penetrating qualities thereof, as well as increasing tosome extent the water-repellency of the image treated therewith. Theuseful quantity of metal soap to be dissolved in the oleic acid or otheroil in general falls within the range of from 0.1 to 5 percent of theweight of solvent. For example, solutions of 0.25 percent of aluminumoleate, 1.0 percent of zinc oleate, and 2.5 percent of magnesium oleatehave all given good results. Stearates and palmitates may be used inplace of oleate soaps if desired.

Example 1 In a typical case a planographic printing plate was used whichcomprised a water-impervious paper base provided by a suitablewater-barrier coating and having on one side a super-calendered coatingof casein and clay in amount of about 2 pounds, dry weight, per 1000square feet of surface and a thin overlying layer, amounting to about1.1 ounces, dry weight, per 1000 square feet of the water-solublesulfonate formed by reacting sodium sulfite and the zinc double salt ofthe reaction product of formaldehyde and 4-diazo-1,1 diphenylamine (seeU. S. Patent No. 2,063,631). This plate was exposed through a negativetransparency to the action of actinic radiation, whereby the exposedareas of the diazo-sulfonate product were rendered insoluble in waterbut did not become unwettable by water. The exposed surface was wipedover with a cotton swab moistened with oleic acid. Next the surface wasswabbed with a pad moistened with water which removed from the unexposedareas the still soluble photosensitive material together with anyoverlying oleic acid. The surface was then wiped over with a swab wetwith a standard lithographic starting solution (Platex manufactured bythe Addressograph-Multigraph Corporation of Cleveland, Ohio). Thereafterthe plate was immediately placed in a rotary offset duplicator where theplate at once satisfactorily picked up ink on the imaged areas, and atonce gave satisfactory black prints therefrom.

Similar results are obtainable by substituting the following for theoleic acid of the foregoing example:

The supercalendered clay-coated paper base of Example 1 was washed witha 3 per cent aqueous solution of the sulfonate formed by reacting sodiumsulfite in aqueous solution with the diazo compound of4-amin0-2',4',6'-trichloro-diphenylamine. The product was dried atmoderate temperature and exposed through a negative to actinicradiation. The surface was wiped over with a cotton swab moistened witholeic acid. Then the surface was swabbed with a pad moistened withwater, which removed from the unexposed areas the still solublephotosensitive material together with any overlying oleic acid. Thesurface was then wiped over with a swab wet with a standard lithographicstarting solution (Platex) and the plate was placed in a rotary onsetduplicator where the imaged areas at once satisfactorily picked up ink,while the background remained satisfactorily clean.

Example 3 A supercalendered clay-coated paper similar to that used inExample 1 was coated with about 3 oz. dry weight, per thousand squarefeet, of the guanidide of the diazo compound of 4-(N-2,6-dichloro-benzyl)-amino-aniline deposited from aqueous medium. The sheetwas dried at moderate temperature and exposed through a negative toradiation from an arc-lamp. The surface was then wiped with a swabmoistened with oleis acid, and was then washed with water. The resultingplate was then wet with a standard lithographic starting solution(Platex) and run in a rotary offset duplicator where the imaged areasimmediately took up ink satisfactorily while the unimaged areas remainedsatisfactorily clean.

I claim:

1. Process which comprises exposing to actinic radiation a paper basehaving a surface sensitized with a coating of a photosensitivecomposition comprising a member of the group consisting of diazocompounds of the general formula RYAI'N2X wherein R stands for a memberof the group consisting of alkyl, aralkyl, aryl and aroyl, Y stands fora member of the group consisting of O, S, NH, N-alkyl, N-aralkyl andN-aryl, Ar stands for aryl and X stands for an equivalent of an anion ofan acid, the amino derivatives of said diazo compounds, the sulfonatesof said diazo compounds, the reaction products of said diazo compoundsand carbonyl compounds and the sulfonates of said reaction prodnets ofsaid diazo compounds and carbonyl compounds; treating the exposedsurface with a nonpenetrating substance from the group consisting ofpowdered water-insoluble metal soaps, powdered water-repellentdyestuffs, oleic acid, vegetable oils, liquid silicones of viscosity ashigh as that of oleic acid, and turpentine polymerized to a highviscosity; washing the surface with water; and printing copies from thetreated surface by the offset printing process.

2. Process as defined in claim 1 in which the composition comprises asulfonate of the diazo compound.

'3. Process as defined in claim 1 in which the composition comprises areaction product of an aldehyde and the diazo compound.

4. Process as defined in claim 1 in which the composition comprises thesulfonate of a reaction product of an aldehyde and the diazo compound.

5. Process as defined in claim 1 in which the composition comprises anamino derivative of the diazo compound.

6. Process as defined in claim 1 in which the composition comprises theguanidide of the diazo compound of 4-(N-2,6-dich1oro-benzyl)-aminoaniline.

'7. Process as defined in claim 1 in which the composition comprises thereaction product of sodium sulfite and the diazo compound of 4- amino-2',4 ,6-trichloro-diphenylamine.

8. Process which comprises exposing to actinic radiation the surface ofa planographic printing plate having a paper base which is sensitized bya photosensitive composition formed by reacting an alkali metal sulfitewith the zinc double salt of the formaldehyde condensation product of 4-diazo-1,1-diphenylamine, moistening the exposed surface With oleic acid,thereafter washing the treated surface with water, and applying aqueouslithographic-solution and lithographic printing-ink to the surface.

References Cited in the file of this patent UNITED STATES PATmVTS NumberName Date 2,463,554 Osinski Mar. 8, 1949 2,515,536 Van Dusen July 18,1950 2,548,537 Kenyon Apr. 10, 1951 2,568,503 Kenyon Sept. 18, 19512,649,373 Neugebauer et al. Aug. 18, 1953 FOREIGN PATENTS Number CountryDate 904,255 France Feb. 19, 1944

1. PROCESS WHICH COMPRISES TO ACTINIC RADIATION A PAPER BASE HAVING ASURFACE SENSITIZED WITH A COATING OF A PHOTOSENSITIVE COMPOSITIONCOMPRISING A MEMBER OF THE GROUP CONSISTING OF DIAZO COMPOUNDS OF THEGENERAL FORMULA R-Y-AR-N2-X WHEREIN R STANDS FOR A MEMBER OF THE GROUPCONSISTING OF ALKYL, ARALKYL ARYL AND AROYL, Y STANDS FOR A MEMBER OFTHE GROUP CONSISTING OF O, S, NH, N-ALKYL, N-ARALKYL AND N-ARYL, ARSTANDS FOR ARYL AND X STANDS FOR AN EQUIVALENT OF AN ANION OF AN ACID,THE AMINO DERIVATIVES OF SAID DIAZO COMPOUNDS, THE SULFONATES OF SAIDDIAZO COMPOUNDS, THE REACTION PRODUCTS OF SAID DIAZO COMPOUNDS ANDCARBONYL COMPOUNDS AND THE SULFONATES OF SAID REACTIONS PRODUCTS OF SAIDDIAZO COMPOUNDS AND CARBONYL COMPOUNDS; TREATING THE EXPOSED SURFACEWITH A NONPENETRATING SUBSTANCE FROM THE GROUP CONSISTING OF POWDEREDWATER-INSOLUBLE METAL SOAPS, POWDERED WATER-REPELLENT DYESTUFFS, OLEICACID, VEGETABLE OILS, LIQUID SILICONES OF VISCOSITY AS HIGH AS THAT OFOLEIC ACID, AND TURPENTINE POLYMERIZED TO A HIGH VISCOSITY; WASHING THESURFACE WITH WATER; AND PRINTING COPIES FROM THE TREATED SURFACE BY THEOFFSET PRINTING PROCESS.